Hybrid powertrains typically include an engine and at least one electric motor/generator used for vehicle propulsion, regenerative vehicle braking or both. The electric motor/generator is also typically used for quickly restarting the engine, to eliminate idle time with the engine running but not providing propulsion. In some hybrid powertrains, the electric motor/generator may restart the engine while also providing vehicle propulsion, so that the electric motor/generator must then have sufficient torque and power capacity to accomplish both simultaneously. If, instead, the full torque or power capacity of the electric motor/generator or motor/generators, or the full electric current or electric power capacity of other components, were used to propel the vehicle, then the extra torque or power from engine starting would be diverted from vehicle propulsion and would produce a momentary disruption in vehicle propulsion.
Electrically-variable transmissions typically have an input member, an output member, and two electric motor/generators connected to different members of planetary gear sets. Including clutches allows one or more electrically-variable modes of operation, fixed speed ratio modes, and an electric-only (battery-powered) mode. Electrically-variable transmissions may improve vehicle fuel economy in a variety of ways, primarily by using one or both of the motor/generators for vehicle braking and using the regenerated energy to power the vehicle electrically, with the engine off. The engine may be turned off at idle, during periods of deceleration and braking, and during periods of low speed or light load operation to eliminate efficiency losses due to engine drag. Braking energy captured via regenerative braking (or electrical energy generated during periods when the engine is operating) is utilized during these engine-off periods. Transient demand for engine torque or power is supplemented by the motor/generators during operation in engine-on modes, allowing for a smaller engine without reducing vehicle performance. Additionally, the electrically-variable modes may allow the engine to be operated at or near the optimal efficiency point for a given power demand.
To transition from an electric-only operating mode to an electrically-variable operating mode, the engine must be restarted. In an electrically-variable transmission, or other two-motor hybrid, this may be accomplished by using one motor/generator or a combination of both motor/generators to restart the engine.